Method for producing a surfactant-free suspension based on nanostructured, hydrophobic particles, and use of the same

ABSTRACT

The present invention describes a process for preparing a surfactant-free suspension including hydrophobic particles and at least 50.01% by weight of water, where hydrophobic, nanostructured particles are suspended in at least one organic solvent miscible with water and having a boiling point below 150° C., and then this concentrated organic suspension is intensively and thoroughly mixed with surfactant-free water.

The present invention relates to a process for preparing asurfactant-free suspension including hydrophobic particles and at least50.01% by weight of water, where hydrophobic, nanostructured particlesare suspended in at least one organic solvent miscible with water andhaving a boiling point below 150° C., and then this concentrated organicsuspension is intensively and thoroughly mixed with surfactant-freewater.

Suspensions of hydrophobic, nanostructured particles are used in theproduction of dirt- and water-repellant coatings on articles. In thecoating process here, hydrophobic particles are applied to the surfaceof the articles, and thus a surface structure with elevations isproduced on the surface of the articles, having dirt- andwater-repellant properties.

The principle of self-cleaning coatings is well known. To achieve goodself-cleaning of a surface, the surface has not only to be veryhydrophobic but must also have some degree of roughness. A suitablecombination of structure and hydrophobic properties permits even smallamounts of water moving on the surface to entrain adherent dirtparticles and clean the surface (WO 96/04123, U.S. Pat. No. 3,354,022).

The prior art of EP 0 933 388 requires an aspect ratio >1 and a surfaceenergy of less than 20 mN/m for these self-cleaning surfaces, the aspectratio being defined here as the quotient which is the ratio of theaverage height of the structure to its average width. The abovementionedcriteria are to be found in the natural world, for example on thesurface of the lotus leaf. The surface of the plant, formed from ahydrophobic waxy material, has elevations separated by up to a few μm.Water droplets essentially contact only these peaks. There are manydescriptions in the literature of water-repellant surfaces of this type.

Swiss patent 268 258 describes a process which generates structuredsurfaces by applying powders, such as kaolin, talc, clay, or silica gel.Oils and resins based on organosilicon compounds are used (examples 1 to6) to secure the powders to the surface.

EP 0 909 747 describes a process for generating a self-cleaning surface.The surface has hydrophobic elevations whose height is from 5 to 200 μm.A surface of this type is produced by applying a dispersion ofpulverulent particles and of an inert material in a siloxane solution,and then curing. The structure-forming particles are secured to thesubstrate by way of an auxiliary medium.

WO 00/58410 concludes that it is technically possible to render thesurfaces of articles artificially self-cleaning. The surface structuresnecessary for this purpose, made from elevations and depressions, have aseparation in the range from 0.1 to 200 μm between the elevations ofthese surface structures, and have an elevation height in the range from0.1 to 100 μm. The materials used for this purpose have to be composedof hydrophobic polymers or of lastingly hydrophobicized material.Release of the particles from the carrier matrix has to be prevented.

The use of hydrophobic materials, such as perfluorinated polymers, toproduce hydrophobic surfaces is known. A further development of thesesurfaces consists in structuring the surfaces in the μm to nm range.U.S. Pat. No. 5,599,489 discloses a process in which a surface can berendered particularly repellant by bombardment with particles ofappropriate size, followed by perfluorination. H. Saito et al. in“Surface Coatings International” 4 (1997), 178 et seq., describe anotherprocess, in which particles made from fluoropolymers are applied tometal surfaces, giving the surfaces thus generated markedly lowerwettability with respect to water, with considerably reduced tendencytoward icing.

The principle of self-cleaning surfaces has been borrowed from thenatural world. Small contact surfaces lower the level of van der Waalsinteraction responsible for adhesion to flat surfaces with low surfaceenergy. For example, the leaves of the lotus plant have elevations madefrom a wax, and these reduce the area of contact with water.

Processes for producing these structured surfaces are likewise known.Besides the use of a master structure to mold these structures in fulldetail by injection molding or embossing processes, there are also knownprocesses which utilize the application of particles to a surface (DE100 22 246 A1).

JP 07328532 A describes a water-repellant film which comprises particleswhose diameter is from 1 nm to 1 mm. The process described first appliesa coating, and then the hydrophobic particles, e.g. AEROSIL®, areapplied. It is important here that the coating is present in uncured oronly partially cured form. Once the coating has been cured, thesuperfluous particles are moved.

BASF describes, in DE 100 49 338, a microstructured, self-cleaningcatalytically active surface, and also a process for its production.This has the advantage that the catalytically active material has beenvery substantially protected from poisoning within the depressions, sothat the overall surface remains catalytically active. This method canbe used to produce heterogeneous catalysts, and when these are used itbecomes unnecessary to rid the product stream of undesirable particleswhich deactivate the catalyst.

Attempts have also recently been made to provide self-cleaning surfaceson textiles. An example of a method found for generating self-cleaningsurfaces is the application of hydrophobic, fumed silicas on textiles.Here, the hydrophobic, fumed silicas are bonded into the polymer matrixof the textile fiber during exposure to a solvent.

DE 101 18 348 describes polymer fibers with self-cleaning surfaces. Inthese, the self-cleaning surface is obtained by

-   -   exposure to a solvent which comprises structure-forming        particles,    -   using the solvent to solvate the surface of the polymer fibers,    -   adhesion of the structure-forming particles to the solvated        surface, and    -   removing the solvent.

A disadvantage of this process is that during processing of the polymerfibers (spinning, knitting, etc.) the structure-forming particles, andtherefore the structure which renders the surface self-cleaning, canbecome damaged or sometimes even be lost entirely, the result being thatthe self-cleaning effect is likewise lost. Another disadvantage is theuse of solvents. High apparatus costs and safety-engineering cost haveto be met here, for reasons associated with the environment.

DE 101 18 346 describes textile sheets with self-cleaning andwater-repellent surface, composed of at least one synthetic and/ornatural textile base material A, and of an artificial, at least to someextent hydrophobic surface with elevations and depressions made fromparticles which have been securely bonded to the base material A withoutadhesives, resins, or coatings. These are obtained by treating the basematerial A with at least one solvent which comprises the undissolvedparticles, and removing the solvent, whereupon at least some of theparticles become securely bonded to the surface of the base material A.However, the disadvantage of this process is very complicated finishingof the textile surfaces. This process requires precise matching of thesolvent to the base material of the textiles. However, in clothing thereare generally mixed fabrics present, and this matching therefore becomesmore complicated. If the matching of the solvents is not precise, theresult can be irreparable damage to parts of the clothing. The surfacestherefore have to be treated prior to tailoring.

All of these coatings have the disadvantage that they are appliedpermanently to the articles and can therefore not be simply removed andreapplied in the event of impairment by scratching or discoloration orany other damage to the surface or surface structure. If this type ofdamage occurs, the article either has to be freed from the surfacestructure by a complicated method and retreated, or has to be disposedof.

WO 00/58410 describes a process for producing detachable coatings withdirt- and water-repellent properties, these being produced byspray-application of hydrophobic alcohols, such as nonacosan-10-ol, orof alkanediols, such as nonacosane-5,10-diol, or of waxes. Thesecoatings can be removed from the articles by strong mechanical forces,e.g. scratching, brushing, or high-pressure water treatment, or bytreatment with water which comprises detergents, which disperse some ofthe structure-formers. A disadvantage here is the strong forces neededfor mechanical removal of the coating, always creating a risk that whenthe coating is removed the article itself will also be damaged.Treatment with water which comprises detergents can likewise lead todamage to the article, depending on its nature.

DE 101 35 157 describes a process for the coating of textiles during adry cleaning procedure, in which structure-forming particles are addedto the cleaning agent. The cleaning agents proposed comprise organicsolvents which are relatively hazardous to health, e.g.trichloroethylene, and the use of these leads to mechanical anchoring ofthe particles to the structure of the textiles. Operation other than inclosed-circuit systems may be legal in some countries but is hazardousto the environment.

DE 28 44 052 describes a process for preparing inverse aqueousdispersions of a hydrophobic silica. This process mixes the hydrophobicsilica and the water intensively and intimately, where appropriate withaddition of a wetting agent. The result is what is known as “dry water”,in which very fine water droplets have been encapsulated by a layer ofsilica. According to the examples presented, the resultant dispersion ispasty to free-flowing.

These pasty dispersions or free-flowing mixtures are unsuitable for usein producing self-cleaning surfaces. WO 00/58410 therefore usessuspensions of hydrophobic particles exclusively based on organicsolvents for producing self-cleaning surfaces. As the self-cleaningsurfaces are formed, relatively large amounts of organic solventstherefore escape.

It was therefore an object of the present invention to provide a processfor producing water-based, surfactant-free suspensions of hydrophobic,nanostructured particles for the production of dirt- and water-repellantcoatings on articles. This coating is intended to be relatively stable,even when the articles are made from sensitive materials. The dirt- andwater-repellent coating is moreover intended to be detachable usingsimple means, i.e. without the use of organic solvents or of strongmechanical forces, such as scouring or scrubbing. The suspension of thehydrophobic, nanostructured particles is intended to be verysubstantially water-based, in order to minimize the content of organicsolvent escaping into the environment.

Surprisingly, it has been found that an aqueous, surfactant-freesuspension of hydrophobic, nanostructured particles can be prepared byfirst suspending the hydrophobic, nanostructured particles in an organicsolvent or in a solvent mixture, and then mixing this concentratedsuspension of particles intensively with surfactant-free water. Applyingthis aqueous, surfactant-free suspension to the surface of an articleand then removing the suspension medium can give coatings with dirt- andwater-repellant properties, which are relatively stable when exposed torain or water spray, but can be removed by simple means. A particularlysurprising fact is that it is possible to produce homogeneous,surfactant-free suspensions of extremely hydrophobic particles in awater-based suspension medium, and that these suspensions can be used togenerate hydrophobic, nanostructured surfaces.

The present invention therefore provides a process for preparing asurfactant-free suspension including hydrophobic particles and at least50.01% by weight of water, where hydrophobic, nanostructured particlesare suspended in at least one organic solvent miscible with water andhaving a boiling point below 150° C., and then this concentrated organicsuspension is intensively and thoroughly mixed with surfactant-freewater.

The invention also provides aqueous suspensions, and the use of thesefor producing detachable, dirt- and water-repellant coatings onarticles. The invention also provides a process for producing adetachable, dirt- and water-repellant coating on articles. During thecoating process, hydrophobic particles are applied to the surface of thearticles and thus a surface structure with elevations is produced on thesurface of the articles, and has dirt- and water-repellant properties,and comprises applying, to at least one surface, a suspension, and thenremoving the suspension medium.

The present invention also provides articles which have, on at least onesurface, a water- and dirt-repellant coating.

The present invention also provides a protective coating, and also animpregnating sprayer which comprises a suspension of the invention.

The present invention describes a process for preparing water-basedsurfactant-free suspensions of hydrophobic, nanostructured particles,which can be used for producing detachable, dirt- and water-repellantcoatings.

The invention has the advantage that a simple method can be used toequip articles with a non-permanent dirt- and water-repellant layer, theonly factor determining the selection of the articles which can beequipped with this coating being compatibility with, and wettability by,the predominantly aqueous suspension medium. Unlike the processes of theprior art, the inventive coating with a detachable, dirt- andwater-repellant layer is relatively non-injurious to the environment andto health, since only water and small amounts of organic solvent, thesebeing used as suspension medium, are released when this coating isapplied. Another advantage of this coating process using the suspensionof the invention is the detachment of the coating by water at slightlyelevated pressure, and this can take place without the use ofdetergents.

The coating produced by the process of the invention is non-permanentand is therefore particularly well suited to be a coating which protectsnew articles and products from contamination, e.g. during transport orin spaces used for sales or presentations. The protective coatings ofthe invention can be detached without difficulty via exposure to liquiddroplets at an elevated pressure.

A feature of the process of the invention for preparing asurfactant-free suspension which comprises hydrophobic particles and atleast 50.01% by weight, preferably at least 60.00% by weight, andparticularly preferably at least 80.00% by weight, of water, is thathydrophobic, nanostructured particles are suspended in at least oneorganic solvent miscible with water and having a boiling point below150° C., and then this concentrated organic suspension is intensivelyand thoroughly mixed with surfactant-free water.

In the first step of the process of the invention, hydrophobic,nanostructured particles are suspended in at least one organic solvent.The organic solvent used is water-miscible and has a boiling point below150° C., preferably below 120° C., particularly preferably below 100° C.The organic solvent used preferably comprises acetone, tetrahydrofuran,or an alcohol which is liquid at room temperature, in particularmethanol, ethanol, n-propanol, or isopropanol. The alcohol used veryparticularly preferably comprises ethanol. However, it can also beadvantageous for the suspension of the invention to comprise a mixtureof these organic solvents.

The particles used in the process of the invention may comprise at leastone material selected from minerals, aluminum oxide, silicates,hydrophobicized silicas, metal oxides, mixed oxides, metal powders,pigments, and polymers. The particles may particularly preferably besilicates, doped silicates, minerals, metal oxides, aluminum oxide,precipitated silicas, or fumed silicas (Aerosils®), or pulverulentpolymers, e.g. spray-dried and agglomerated emulsions, or cryogenicallyground PTFE. The hydrophobic particles used particularly preferablycomprise silicas.

The average diameter of the particles used in the process of theinvention is preferably from 0.01 to 100 μm, particularly preferablyfrom 0.02 to 50 μm, and very particularly preferably from 0.05 to 30 μm.However, other particles which are suitable are those which under dryconditions, starting from primary particles, combine to giveagglomerates or aggregates with sizes from 0.2 to 100 μm.

It can be advantageous for the particles used in the process of theinvention to have a structured surface. Preference is given to the useof particles which have an irregular surface nanostructure, i.e. in therange from 1 to 1000 nm, preferably from 2 to 750 nm, and veryparticularly preferably from 10 to 100 nm. A fine structure meansstructures whose heights, widths, and separations are within the rangesmentioned. These nanostructured particles preferably comprise at leastone compound selected from fumed silica, precipitated silicas, aluminumoxide, silicon dioxide, titanium dioxide, zirconium dioxide, andpulverulent polymers.

The hydrophobic properties of the particles used in the process of theinvention may be inherently present through the materials used for theparticles, as is the case, for example, with polytetrafluoroethylene(PTFE). However, it is also possible to use hydrophobic particles whichhave hydrophobic properties after suitable treatment, e.g. particlestreated with at least one compound selected from the group consisting ofthe alkylsilanes, the fluoroalkylsilanes, and the disilazanes.Particularly suitable particles are hydrophobicized fumed silicas, knownas Aerosils®. Examples of hydrophobic particles are Aerosil® VPR 411,Aerosil® VP LE 8241 and Aerosil® R 8200. Examples of particleshydrophobicized by treatment with perfluoroalkylsilane followed byheat-conditioning are Aeroperl 90/30®, Sipemat silica 350®, aluminumoxide C®, zirconium silicate, and vanadium-doped, or Aeroperl P 25/20®.

In the process of the invention, the hydrophobic, nanostructuredparticles are suspended homogeneously in an organic solvent or in amixture thereof. For this, use is preferably made of from 0.01 to 10.00%by weight, with preference from 0.05 to 5.00% by weight, and veryparticularly preferably from 0.10 to 2.50% by weight, of hydrophobicparticles based on the inventive suspension to be prepared. The amountof organic solvent used for this purpose is preferably from 0.10 to49.98% by weight of the inventive suspension to be prepared, butparticularly preferably from 0.50 to 49.98% by weight, and veryparticularly preferably from 4.00 to 49.98% by weight.

The highly concentrated suspension of the hydrophobic, nanostructuredparticles in an organic solvent or in an organic solvent mixture is thenintensively and thoroughly mixed with surfactant-free water, in afurther step of the process of the invention. In one preferredembodiment of the process of the invention, the organic suspension ofthe hydrophobic, nanostructured particles is added, by means of ametering apparatus, into a mixing chamber in which it is vigorously andthoroughly mixed with the surfactant-free water, which forms an initialcharge. In another embodiment of the process of the invention, thesurfactant-free water is added, by means of a metering apparatus, into amixing chamber in which it is vigorously and thoroughly mixed with theorganic suspension of the hydrophobic, nanostructured particles, whichforms an initial charge.

The water used in the process of the invention is preferablydemineralized water, but particularly preferably distilled water. Tobreak down any agglomerates present it is advantageous to suspend thehydrophobic particles by introducing high levels of shear energy, forexample by means of a dissolver disc. It can also be advantageous to useinternals which increase turbulence, for example baffles or otherobstructions which avoid any formation of standing waves. The Reynoldsnumber is preferably above 2320. The Richardson number calculates theratio between the magnitude of thermal stratification and of themagnitude of shear in a flow. It is therefore a number which measuresthe ratio of the variables responsible for the spread or decay ofturbulence. The Richardson number relates to a local gradient at onepoint in the flow, and for the process of the invention the Richardsonnumber is not more than 0.25. An example of a definition of theRichardson number is found in H. Kobus (lecture script“Gewässerhydraulik” [Hydraulics of natural water systems] Lehrstuhl fürHydraulik und Grundwasser, Institut für Wasserbau [Department ofHydrogeology, Institute of Hydraulic Engineering], StuttgartUniversity).

The aqueous suspension of the invention may be prepared by the processof the invention, and preferably comprises from 0.01 to 10.00% byweight, with preference from 0.05 to 5.00% by weight, and veryparticularly preferably from 0.10 to 2.50% by weight, of hydrophobic,nanostructured particles. The suspension of the invention comprises from0.10 to 49.98% by weight, particularly preferably from 0.50 to 49.98% byweight, and very particularly preferably from 4.00 to 49.98% by weight,of organic solvent, based on the inventive suspension to be prepared.The organic solvent used preferably comprises acetone, tetrahydrofuran,or an alcohol which is liquid at room temperature, in particularmethanol, ethanol, n-propanol, or isopropanol. The alcohol usedparticularly preferably comprises ethanol. However, it can also beadvantageous for the suspension of the invention to comprise a mixtureof these organic solvents. The suspension of the invention comprises atleast 50.01% by weight, preferably 60.00% by weight, and particularlypreferably 80.00% by weight, of water.

The shelf life of the suspension of the invention becomes longer as thecontent of organic solvent increases. However, shaking prior to use ofthe suspension of the invention is always advisable. Since noagglomeration of the particles occurs, the shaking is sufficient toensure renewed homogeneous distribution of the particles in thesuspension of the invention.

The suspension of the invention may be used for producing detachable,dirt- and water-repellant coatings on articles. Another process of theinvention describes the production of a detachable, dirt- andwater-repellant coating on articles, where, during the coating process,hydrophobic particles are applied to the surface of the articles andthus a surface structure with elevations is produced on the surface ofthe articles, and has dirt- and water-repellant properties, where asuspension of the invention is applied to at least one surface of anarticle, and the suspension medium is then removed.

The suspension may be applied to at least one surface of an article in amanner known to the skilled worker. The suspension of the invention ispreferably applied by dipping the article into the suspension of theinvention, and then permitting the material to run off, or byspray-application of the suspension of the invention to the article.Surprisingly, spray-application of the suspension can in particulargenerate especially durable coatings which have relatively lowsusceptibility to scratching and to abrasion, in particular on polymersurfaces. A possible reason for this is that a material such as alcoholin water facilitates charge equilibration on polymer surfaces, and thusreduces the level of local electrical fields. As a result of thisreduction, the particles find it easier to anchor into the surfaceroughness which is always present. In the spraying procedure, theparticles become even more securely embedded into the surface roughnesspresent on the polymer matrix, due to their kinetic energy. Thespray-application of the suspension of the invention preferably uses aspray apparatus which has a nozzle of diameter from 0.05 to 2 mm,preferably of diameter from 0.1 to 0.9 mm. It can be advantageous forthe Manning coefficient in the tubes of the spray apparatus to assumevalues below 100. According to Garbrecht (seetu-harburg.de/www/vorlesung-/Script-HydromechWS01-5-6.pdf on theworldwide web), the Manning coefficient can be derived as follows fromthe wall roughness k:Manning coefficient=26/k ^(1/6).

In one particular embodiment, the suspension of the invention is appliedby means of a sprayer. Containers which may be used arepropellant-charged pressure containers or “pumpspray bottles”, which canbe used without adding propellants and without damaging the environment.

The suspension medium for the suspension of the invention is a mixtureof water and organic solvent and is advantageously removed byevaporation or volatilization, and the evaporation or volatilization maybe accelerated by using elevated temperatures or by using subatmosphericpressure or vacuum.

The coatings produced by the process of the invention are self-cleaningand water-repellant, and preferably have elevations, formed by theparticles and, where appropriate, by the fine structure of theparticles, whose average height is from 50 nm to 25 μm and whose averageseparation is not more than 25 μm, preferably whose average height isfrom 100 nm to 10 μm and/or whose average separation is not more than 10μm, and very particularly preferably whose average height is from 100 nmto 4 μm and/or whose average separation is not more than 4 μm. Thecoatings produced by the process of the invention very particularlypreferably have elevations whose average height is from 0.05 μm to 0.5μm and whose average separation is not more than 0.5 μm. For thepurposes of the present invention, the average separation is theseparation of the highest elevation of an elevation from the mostadjacent highest elevation. If an elevation has the shape of a cone, thetip of the cone is the highest elevation of the elevation. If theelevation is a rectangular parallelepiped, the uppermost surface of therectangular parallelepiped is the highest elevation of the elevation.The average width of the elevations is preferably from 40 nm to 25 μm,preferably from 50 nm to 10 μm, and very particularly preferably from0.05 to 0.5 μm. The average width of the elevations is measured at halfof the height of the elevations, and averaged across the smallest andthe largest width. The average width of a cone or of a cylinder istherefore the diameter of the cylinder or cone at half its height. Theaverage width for a cube is the average derived from the length of theface plus the length of the face diagonals.

The coatings produced by the process of the invention preferably have acontact angle for water greater than 130°, preferably greater than 140°,particularly preferably greater than 145°.

The process of the invention may be used to produce articles which, onat least one surface, have a water- and dirt-repellant coating. Thearticles or the surfaces to be coated may be composed of a very widevariety of substances, e.g. metal, plastic, polymer, wood, ceramics, orglass.

The protective coating of the invention may be produced by the processof the invention for producing detachable, water- and dirt-repellantcoatings on articles. This protective coating of the invention may beremoved from the surface of the article by subjection to impact from aliquid with momentum greater than 12 mNs, for example by treatment withliquid droplets which impact the surface with momentum greater than 12mNs. This value corresponds to the momentum of an average raindropfalling toward the earth at twice the velocity normally encountered inthe natural world. The protective coatings of the invention maytherefore be detached from the coated article by a simple method using awater jet. The protective coating can preferably be detached from thearticle by means of a water jet whose momentum is greater than 12 mNsand less than 60 mNs, preferably less than 30 mNs and greater than 15mNs. No detergents are needed in the water in order to detach theprotective coating.

The protective coating of the invention may be used to protect articles,e.g. machine tools or means of transport, e.g. ships, aircraft, cycles,or motor vehicles, e.g. cars, buses, trucks, or motorcycles, orproducts, from soiling during transport or in spaces used for sales orpresentations. Other claimed applications are those for the protectionof packaged products, i.e. products which comprise packaging material,for example for transport, and which have to be placed in intermediatestorage outdoors where dirt can represent a risk.

The protective coating of the invention may moreover be used to protectarticles, films or textiles from high levels of contamination by dirtand/or water during outdoor activities, during leisure or during work,in particular in the case of articles and textiles for ski sports,alpine sports, motor sports, motorcycle sports, motocross sports, andsailing sports. The protective coating of the invention may be usedadvantageously for technical textiles and fabrics used for textilebuildings, selected from tents, awnings, umbrellas, tablecloths,cabriolet covers, and workwear.

Impregnating sprays may also comprise the suspension of the invention.The sprays may be used, for example, to provide detachable dirt- andwater-repellant coatings for garden furniture, car wheel rims, carpaints, showers, tiles, sanitary surfaces in general, laundries, and thelike. An active ingredient preferably present in this impregnating sprayis an inventive suspension of hydrophobic particles in a predominantlyaqueous medium, as described above.

A description of the process of the invention, and also of their use, isgiven below, but without any intention that the invention be restrictedthereto.

EXAMPLE 1 Preparation of Suspensions of the Invention

Aerosil® VP LE 8241 was suspended in ethanol in a Ultra-Turrax.Demineralized water was added to, and vigorously and thoroughly mixedwith, the highly concentrated suspension of Aerosil® in ethanol, usingan Ultra-Turrax. The suspension of the invention was then drawn off intospray bottles.

Table 1 shows the experimental parameters of Example 1.

TABLE 1 Experiment number 1 2 3 Amount of demineralized 92 79 59 water(in. % by weight) Amount of ethanol 7 20 40 (in % by weight) Amount ofAerosil ® 1 1 1 VP LE 8241 (in % by weight)

EXAMPLE 2 Spray-Application of Suspension of the Invention to Surfaces

The suspensions of the invention were prepared as in Example 1 andapplied, by means of a spray bottle, to the surface of the testspecimens (polymethyl methacrylate (PMMA) plaques of thickness 2 mm),and the solvent or solvent mixture was then removed overnight byevaporation at room temperature. The surfaces were then firstcharacterized visually, and recorded as +++ for all three of thesuspensions. +++ means that there is almost complete formation of waterdroplets. The roll-off angle is below 10°.

1. A surfactant-free aqueous suspension, comprising water, hydrophobicnanostructured particles and an organic solvent; wherein thesurfactant-free aqueous suspension is prepared by a process comprising:suspending the hydrophobic nanostructured particles in at least oneorganic solvent miscible with water and having a boiling point below150° C. to form a concentrated organic suspension, and then mixing theconcentrated organic suspension with surfactant-free water to form thesurfactant-free aqueous suspension; wherein the surfactant-free aqueoussuspension comprises at least 50.01% by weight of water; wherein thehydrophobic nanostructured particle has an average diameter of from 30to 100 μm and is present in an amount of from 0.01 to 1% by weight basedon the total weight of the surfactant-free aqueous suspension; andwherein the hydrophobic nanostructured particles are particles ofpolytetrafluoroethylene; and wherein the hydrophobic, nanostructuredparticles are not agglomerated.
 2. The surfactant-free aqueoussuspension as claimed in claim 1, comprising from 0.1 to 49.9% by weightof at least one alcohol, wherein the alcohol is miscible with water andhas a boiling point below 150° C.
 3. The surfactant-free aqueoussuspension as claimed in claim 1, comprising at least 60% by weightwater.
 4. The surfactant-free aqueous suspension of claim 1, wherein theorganic solvent is at least one of acetone and tetrahydrofuran.
 5. Thesurfactant-free aqueous suspension as claimed in claim 1, wherein theorganic solvent is at least one selected from the group consisting ofmethanol, ethanol, n-propanol and isopropanol.
 6. The surfactant-freeaqueous suspension as claimed in claim 1, wherein the hydrophobic,nanostructured particles have an irregular surface nanostructuredcomprising a fine structure with features of from 10 to 100 nm.
 7. Thesurfactant-free aqueous suspension as claimed in claim 1, wherein thesurfactant-free aqueous suspension consists of water, the hydrophobicnanostructured particles and the organic solvent.
 8. The surfactant-freeaqueous suspension as claimed in claim 1, wherein the hydrophobicnanostructured particles are particles of a cryogenically groundpolytetrafluoroethylene.
 9. A process for producing a detachable, dirt-and water-repellant coating on articles, where, during a coatingprocess, hydrophobic particles are applied to the surface of thearticles and thus a surface structured with elevations is produced onthe surface of the articles, and has dirt- and water-repellantproperties, comprising applying, to at least one surface, thesurfactant-free aqueous suspension as claimed in claim 1, and thenremoving the water and the organic solvent.
 10. The process as claimedin claim 9, wherein the application of the surfactant-free aqueoussuspension to at least one surface of an article takes place by dippingthe article into the surfactant-free aqueous suspension.
 11. The processas claimed in claim 9, wherein the application of the surfactant-freeaqueous suspension to at least one surface of an article takes place byspray-application of the surfactant-free aqueous suspension.
 12. Theprocess as claimed in claim 11, wherein a spray is used to apply thesurfactant-free aqueous suspension.
 13. The process of claim 9, furthercomprising: after removing the water and the organic solvent, removingthe coating from the article by impact with a liquid having a momentumgreater than 12 mNs.
 14. The method as claimed in claim 13, wherein theliquid having a momentum of greater than 12 mNs does not contain adetergent.